Nonaqueous electrolyte secondary cell and positive electrode active material

ABSTRACT

A positive active material for nonaqueous electrolyte secondary batteries which has a higher capacity and improved thermal stability in a charged state and is less expensive compared to the current active material of LiCoO 2  is provided by a lithium compound oxide having the formula:  
     Li a Ni b Co c Mn d M e O 2    (1)  
     where M stands for one or two of W and Mo,  
     0.90≦a≦1.15, 0&lt;b&lt;0.99, 0&lt;c≦0.5, 0&lt;d≦0.5,  
     0&lt;c+d≦0.9, 0.01≦e≦0.1, and b+c+d+e=1,  
     the lithium compound oxide giving an X-ray diffraction pattern including a diffraction peak or peaks assigned to a compound oxide of Li and W and/or a compound oxide of Li and Mo, in addition to main diffraction peaks assigned to a hexagonal crystal structure.

TECHNICAL FIELD

[0001] The present invention relates to a positive active material fornonaqueous electrolyte secondary batteries useful as a power source inportable electronic or communications equipment, electric cars, and thelike, and to a nonaqueous electrolyte secondary battery using thepositive active material.

BACKGROUND ART

[0002] Lithium ion secondary batteries, which are one class ofnonaqueous electrode secondary batteries, have advantages including ahigh voltage, a high energy density, and a low self discharge, and theyhave become indispensable as a power source for portable electronic orcommunications equipment such as mobile phones, laptop personalcomputers, camcorders, and the like.

[0003] Lithium ion secondary batteries which are currently in practicaluse are 4V-grade batteries in which a carbon material such as graphiteis used for the negative electrode, LiCoO₂ (lithium cobaltate) is usedas an active material for the positive electrode, and a nonaqueoussolution of a lithium salt in an organic solvent is used as theelectrolytic solution. When these batteries are charged, the followingreaction occurs:

Charge reaction: LiCoO₂+nC₆→Li_((1−n))CoO₂+nLiC₆.

[0004] A high voltage of at least 4.8 V is required for full (100%)charge of the batteries. However, such a high voltage may causedecomposition of the electrolytic solution and adversely affect thereversibility of charge and discharge reactions, resulting in a loss ofthe cycle life of the secondary batteries. Therefore, in practice, themaximum voltage is limited to 4.1-4.2 V. Thus, the positive activematerial is utilized in a stable region where the value of “n” in theabove reaction is around 0.5, so the positive active material as chargedcan be expressed approximately as Li_(0.5)CoO₂.

[0005] As the performance of portable electronic or communicationsequipment is increased, it is increasingly required for secondarybatteries to have a high energy density with a small size and a lightweight. On the other hand, in the field of large-sized secondarybatteries for use in electric cars which are under development with aview of maintaining the global environment, there is a demand forsecondary batteries which not only have a high energy density but alsoare safe.

[0006] In addition, the costs of secondary batteries are important,particularly for large-sized secondary batteries. The use of anexpensive cobalt compound whose resources are limited as a positiveactive material necessarily adds to the cost of the above-describedpractical lithium ion secondary batteries. The high cost of lithium ionsecondary batteries is a major cause when they are precluded from beingmounted in electric cars.

[0007] It is well known that LiNiO₂ (lithium nickelate) can also be usedas a positive active material for lithium ion secondary batteries. LikeLiCoO₂, LiNiO₂ has a layered, hexagonal crystal structure and allowslithium (Li) ions to be intercalated and deintercalated between layersof the crystal structure. The charge reaction occurring when LiNiO₂ isused as a positive active material is basically the same as theabove-described charge reaction for LiCoO₂. However, LiNiO₂ can becharged in a stable manner until the value of “n” in the above chargereaction formula reaches around 0.7. Thus, in this case, the positiveactive material as charged can be expressed approximately asLi_(0.3)NiO₂, thereby constituting a positive electrode of a highercapacity.

[0008] Compared to LiCoO₂, LiNiO₂ has the advantages of being lessexpensive and being capable of making a secondary battery of highercapacity. However, LiNiO₂ has the problem that its crystal structuretends to be broken during charging and discharging, thereby adverselyaffecting the cycle properties of the secondary battery. In addition, ina secondary battery having a positive active material of LiNiO₂, anexothermic decomposition reaction may occur when the charged positiveactive material is exposed to a high temperature in the presence of theelectrolytic solution, whereby the active material is converted into acompound approximately expressed as Li₂Ni₈O₁₀ and oxygen is liberated.The liberated, active oxygen may react with the electrolytic solution oranother component, or serve as combustion-promoting oxygen. As a result,there may be a risk of igniting the battery itself in some cases. Thus,a secondary battery using LiNiO₂ as a positive active material has poorthermal stability, and this positive active material could not be usedin practical batteries.

[0009] As an attempt to improve the cycle properties of a secondarybattery using LiNiO₂ as a positive active material, stabilizing thecrystal structure of this compound by replacing part of Ni by anotherelement such as Co was investigated, as described in Solid State Ionics,90, 83 (1996). This approach makes it possible to considerably improvethe cycle properties.

[0010] On the other hand, with respect to the thermal stability of asecondary battery using LiNiO₂ as a positive active material, it wasreported in the 40th Symposium on Batteries in Japan (1999),Presentation Number 1C12 that when part of Ni is replaced by Co+Mn, thethermal stability can be improved as the amount of replacing Coincreases or the Ni content decreases with a certain amount of replacingMn. With this approach, however, it is difficult to improve the thermalstability to the same level as that of LiCoO₂, although an initialcapacity surpassing that of LiCoO₂ can be obtained.

[0011] A LiCoO₂-based positive active material containing at least oneelement selected from Cu, Zn, Nb, Mo, and W is described in JP-A06-283174. Although it is explained therein that the positive activematerial has a high capacity and good cycle properties, the cycleproperties are measured with only ten cycles and do not yet reach alevel sufficient for practical use.

[0012] As discussed above, with LiNiO₂-based materials which arepositive active materials less expensive than LiCoO₂, although it ispossible to attain a high capacity surpassing that attainable withLiCoO₂, the thermal stability of the positive active materials in theircharged state is poor, and it is difficult to improve the thermalstability to the same level as that of LiCoO₂. Thus, none of thesematerials have been improved in both initial capacity and thermalstability.

DISCLOSURE OF THE INVENTION

[0013] It is an object of the present invention to develop aLiNiO₂-based positive active material for use in nonaqueous electrolytesecondary batteries which has an initial capacity higher than that ofLiCoO₂ and which is improved in thermal stability in a charged state atleast to the same level as LiCoO₂, thereby making it possible to providenonaqueous electrolyte secondary batteries which are less expensive andhave better performance than the current practical lithium ion secondarybatteries.

[0014] The present inventors found that the thermal stability of aLiNiO₂-based positive active material can be improved at least to thesame level as that of LiCoO₂ by replacing part of the Ni in LiNiO₂ by Coand Mn and further by one or both of W and Mo. Although the invention isnot intended to be bound by a specific theory, it is presumed that theimprovement in thermal stability results from suppression of oxygenliberation which is caused by decomposition of the positive activematerial during charging and also from shifting the decompositiontemperature to a higher temperature.

[0015] The present invention is a positive active material for use innonaqueous electrolyte secondary batteries, characterized in that it iscomprised of a lithium compound oxide (compound oxide of lithium) of theformula:

Li_(a)Ni_(b)Co_(c)Mn_(d)M_(e)O₂   (1)

[0016] where M stands for one or two of W and Mo,

[0017] 0.90≦a≦1.15, 0<b<0.99, 0<c≦0.5, 0<d≦0.5,

[0018] 0<c+d≦0.9, 0.01≦e≦0.1, and b+c+d+e=1,

[0019] and in that the lithium compound oxide gives an X-ray diffractionpattern including a diffraction peak or peaks assigned to a compoundoxide of Li and W and/or a compound oxide of Li and Mo, in addition tomain diffraction peaks assigned to a hexagonal crystal structure.

[0020] The present invention also relates to a nonaqueous electrolytesecondary battery comprising a negative electrode comprised of lithiummetal or a substance capable of absorbing and desorbing Li or Li ionsand a positive electrode comprised of the above-described positiveactive material.

[0021] The positive active material for nonaqueous electrolyte secondarybatteries having a composition shown by the above formula (1) accordingto the present invention has significantly improved thermal stability ina charged state. As can be seen from the DSC (differential scanningcalorimeter) diagrams given in the examples described below, theimproved thermal stability is not only superior to that of LiNiO₂, butis also superior to that of conventional thermally-stabilizedLiNiO₂-based positive active materials in which part of Ni is replacedby Co and Mn, and is even superior to that of LiCoO₂ which is inherentlythermally stable. Thus, addition of an extremely small amount of Wand/or Mo provides a significant improvement in thermal stabilitywithout a significant decrease in initial capacity. The effect of Wand/or Mo on improvement in thermal stability is not known and has beenfirst found by the present inventors. As a result, the present inventionmakes it possible to manufacture nonaqueous electrolyte secondarybatteries having both good thermal stability and good initial capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 shows an X-ray diffraction pattern of a lithium compoundoxide obtained in Example 1 as a positive active material according tothe present invention (in the upper half) and a partial enlarged view ofthe upper chart (in the lower half);

[0023]FIG. 2 shows an X-ray diffraction pattern of a lithium compoundoxide obtained in Example 2 as a positive active material according tothe present invention (in the upper half) and a partial enlarged view ofthe upper chart (in the lower half);

[0024]FIG. 3 shows an X-ray diffraction pattern of a lithium compoundoxide obtained in Example 3 as a positive active material according tothe present invention (in the upper half) and a partial enlarged view ofthe upper chart (in the lower half);

[0025]FIG. 4 shows an X-ray diffraction pattern of a lithium compoundoxide obtained in Comparative Example 1 as a positive active material(in the upper half) and a partial enlarged view of the upper chart (inthe lower half);

[0026]FIG. 5 shows an X-ray diffraction pattern of a lithium nickelateobtained in Comparative Example 2 as a positive active material (in theupper half) and a partial enlarged view of the upper chart (in the lowerhalf);

[0027]FIG. 6 shows an X-ray diffraction pattern of a lithium cobaltateobtained in Comparative Example 3 as a positive active material (in theupper half) and a partial enlarged view of the upper chart (in the lowerhalf);

[0028]FIG. 7 shows DSC diagrams of compound oxides in a charged statewhich were obtained in Examples 1 to 3 and Comparative Examples 1 to 5as positive active materials; and

[0029]FIG. 8 is a schematic illustration showing the structure of acoin-shaped battery assembled in the examples for a battery test.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0030] The positive active material for nonaqueous secondary batteriesaccording to the present invention has a composition as shown in theabove formula (1).

[0031] In compositional formula (1), the molar ratio of Li, “a”, isbetween 0.90 and 1.15. If the value of “a” is less than 0.90 or greaterthan 1.15, Ni or other transitional elements may enter the 3b sites (Lisites) of the layered hexagonal crystal lattice of the lithium compoundoxide, thereby causing the discharge capacity of the material todecrease. The value of “a” is preferably between 0.95 and 1.10 and morepreferably between 0.99 and 1.10.

[0032] The sum of the molar ratios of the metals other than Li (i.e.,b+c+d+e) is 1.

[0033] Nickel (Ni) serves to form the hexagonal crystallographicskeleton of LiNiO₂, which provides a high discharge capacity. The molarratio of Ni, “b”, is the remainder of the sum of the molar ratios of themetals other than Li [i.e., b=1−(c+d+e)]. The value of “b” is greaterthan 0 and less than 0.99.

[0034] Cobalt (Co) and manganese (Mn) are present in order to improvethe thermal stability of LiNiO₂. The molar ratio of Co, “c”, and that ofMn, “d”, are both greater than 0 and not greater than 0.5. The value ofthe sum of (c+d) is greater than 0 and not greater than 0.9. For thepurpose of improving thermal stability, it is advantageous to add bothCo and Mn. However, if the value of either “c” or “d” exceeds 0.5 or thevalue of the sum of (c+d) exceeds 0.9, the proportion of the LiNiO₂skeleton which exhibits a high capacity decreases so as to cause anextreme decrease in the discharge capacity. Preferably, the value of “c”is at most 0.4, the value of “d” is at most 0.3, and the value of thesum of (c+d) is at most 0.7.

[0035] In order to ensure that the thermal stability is improvedsufficiently, it is preferred that Co and Mn be added as follows:c≧0.05, d≧0.01. Most preferably, the value of“c” is between 0.10 and0.35, the value of“d” is between 0.05 and 0.30, and the value of the sumof (c+d) is between 0.25 and 0.65.

[0036] The addition of Co and Mn is not enough to improve the thermalstability of LiNiO₂ sufficiently. In accordance with the presentinvention, the thermal stability of LiNiO₂ is significantly improved byfurther adding one or both of tungsten (W) and molybdenum (Mo). In orderto attain this effect, the value of the molar ratio of M (which standsfor W and/or Mo), “e”, is between 0.01 and 0.1. If the value of “e” isless than 0.01, the thermal stability is not improved sufficiently. Ifit is greater than 0.1, the discharge capacity is deteriorated. Thevalue of “e” is preferably less than 0.05 and more preferably at least0.02 and less than 0.05. Thus, W and/or Mo exhibits a significantthermal stability-improving effect when added in a very small amount.

[0037] A positive active material comprised of a lithium compound oxideof the above formula (1) according to the present invention can beprepared by any appropriate method. A common method comprises mixingoxides of the individual metal elements or their precursors (i.e.,substances capable of forming the desired metal oxides by decompositionor oxidation) at a predetermined ratio as uniformly as possible andcalcining the resulting mixture in an oxidizing atmosphere. This methodis described below more specifically, but it should be understood thatthe positive active material according to the present invention can beprepared by a method other than the described one.

[0038] Ni, Co, and Mn can be coprecipitated as carbonate salts, forexample, to give a compound carbonate salt in which these metal elementsare mixed uniformly at their atomic level. Specifically, an aqueoussolution containing at least one water-soluble compound of each of thesemetals (e.g., an aqueous solution containing sulfates of these metals)at a predetermined atomic ratio is mixed with an alkaline solution ofammonium hydrogen carbonate at room temperature or under warming,thereby causing these metals (metal ions) to coprecipitate as carbonatesalts anid form a compound carbonate of Ni, Co, and Mn . This reactionis preferably carried out by adding to a reactor the aqueous solution ofthe metal compounds and that of ammonium hydrogen carbonate both insmall portions simultaneously or alternatingly in order to facilitate auniform crystal growth. The resulting compound carbonate may beconverted into the corresponding compound oxide by heating at atemperature of 300-900° C. to cause thermal decomposition fordecarboxylation. The metal compounds used as starting materials are notlimited to sulfates, and other appropriate compounds such as chlorides,nitrates, and acetates, which are soluble in water or in an acidsolution, may be used.

[0039] The compound carbonate of Ni, Co, and Mn obtained by theabove-described method or the compound oxide thereof obtained from thecompound carbonate by heating is then uniformly mixed with a Li sourceand a source or sources of W and/or Mo (e.g., using an appropriatemixing machine), and the mixture is calcined in an oxidizing atmosphereto obtain a lithium compound oxide serving as a positive active materialaccording to the present invention. The calcination is carried out undersuch conditions that the Li source reacts with the sources of the otherconstituent metals (which are all transition metals) to form a compoundoxide of Li with the transition metals.

[0040] As the Li source, a lithium compound is more suitable thanlithium metal, which is too active. Useful lithium compounds includelithium hydroxide (anhydrous or monohydrate), lithium carbonate, lithiumnitrate, lithium oxide, and the like. The source of each of W and Mo maybe either the metal itself or a metal compound such as an oxide,carbide, or chloride. Alternatively, W and/or Mo may be introduced intothe above-described compound carbonate by coprecipitation as a carbonatefrom a soluble compound along with Ni, Mn, and Co.

[0041] The metal sources present in the mixture to be calcined have anincreased reactivity as their particle diameters decrease. In thisrespect, the compound carbonate or compound oxide of Ni, Co, and Mn (andoptionally W and/or Mo) preferably has an average particle diameter offrom 6 to 20 μm, while the Li source compound and the sources of Wand/or Mo (if any is used) preferably have an average particle diameterof from 1 to 20 μm. Therefore, each source may be pulverized and/orclassified (sieved) as required.

[0042] The temperature at which the mixture is calcined is determinedsuch that the Li source reacts with the sources of the transition metalsto form a compound oxide of Li with the transition metals. A preferablecalcination temperature is normally in the range of 600-1100° C., andmore preferably in the range of 800-1050° C. The calcination atmosphereis an oxidizing atmosphere and preferably an atmosphere having an oxygenconcentration higher than that of air, and even a pure oxygen atmospheremay be used. The calcination is performed until the reaction between Liand the transition metals is completed. The duration of calcination isnormally at least a few hours depending on the temperature andatmosphere for calcination.

[0043] The positive active material (lithium compound oxide) accordingto the present invention, which is a product obtained by calcination,has a crystal structure in which the basic crystallographic skeleton ishexagonal, which is characteristic of LiCoO₂ and LiNiO₂. Thus, the 3asites of the hexagonal lattices are occupied by Ni and Co, part of whichis further replaced by Mn.

[0044] The state of W and/or Mo as additive elements by which thepresent invention is characterized has not been completely elucidated,but in an X-ray diffraction pattern of the active material, one or morediffraction peaks assigned to a compound oxide of Li with Mo or W can beconfirmed. Therefore, at least part of W and/or Mo crystallizes out as acompound salt with Li to form different phases from the skeletalhexagonal crystals.

[0045] Thus, the lithium compound oxide according to the presentinvention is characterized in that it produces an X-ray diffractionpattern which includes, in addition to main diffraction peaks assignedto the hexagonal crystal structure of the basic skeleton, one or morediffraction peaks assigned to a compound oxide of Li and W and/or acompound oxide of Li and Mo (the latter peaks being hereinafter referredto as secondary diffraction peaks in some places). With a positiveactive material which does not produce such a secondary diffractionpeak, an improved thermal stability cannot be obtained even if itcontains W or Mo.

[0046] Although the particular compounds for a compound oxide of Li andW and that of Li and Mo are not limited, they are typically Li₂WO₄ andLi₄MoO₅ both having a rhombohedral crystal form. Thus, typically, one ormore diffraction peaks assigned to such a rhombohedral crystal appear inan X-ray diffraction pattern of a positive active material according tothe present invention.

[0047] A positive active material according to the present invention canbe used to produce positive electrodes in a conventional manner. Amixture for making positive electrodes (composition for forming positiveelectrodes) which predominantly comprises the positive active materialin powdered form is usually prepared. The mixture usually contains abinder and a conducting additive in addition to the positive activematerial. A positive electrode can be produced by forming a paste from amixture containing these constituents with a small amount of a solventand applying the paste to an electrode substrate serving as a currentcollector to form a thin layer of the paste, which is then dried,optionally after compacting by rolling or similar technique. Instead ofthe paste being applied, the paste may be preformed into a sheet, whichis then press-bonded to an electrode substrate and dried to produce apositive electrode. Other methods may be employed.

[0048] The binder is not critical unless it is attacked by thenonaqueous electrolytic solution of a battery, and it is generally afluoroplastic. The conducting additive is not always necessary, but itis usually added since the electric conductivity of the positive activematerial according to the present invention is not considerably high. Asa conducting additive, a carbon powder such as acetylene black isgenerally used. The electrode substrate may be made of a metal such asaluminum or a stainless steel, and it may be either a solid sheet or ina porous form such as a perforated sheet or mesh. The substrate may be avery thin sheet such as a foil.

[0049] When a nonaqueous electrolyte secondary battery according to thepresent invention is assembled using a positive electrode produced froma positive active material according to the present invention, the otherelements of the battery such as a negative electrode, electrolyticsolution, and separator are not limited to particular forms. Anonaqueous electrolyte secondary battery is represented by a lithiumsecondary battery, and a positive electrode produced from a positiveactive material according to the present invention is suitable for usein a lithium secondary battery, although, in principle, it can be usedin other nonaqueous electrolyte secondary batteries.

[0050] In the case of a lithium secondary battery, a negative electrodefor nonaqueous electrolyte secondary batteries is comprised of eitherlithium metal or a substance capable of reversibly absorbing anddesorbing Li or Li ions. Lithium metal can be used to form a negativeelectrode for an experimental battery, but it is inevitably accompaniedby a loss of cycle life due to precipitation of lithium metal indendrites during charging. Therefore, in a battery for practical use,the negative electrode is normally formed from a substance capable ofreversibly absorbing and desorbing Li or Li ions. Examples of such asubstance for negative electrodes include carbonaceous materialsincluding pyrolytic carbon, coke (pitch coke, needle coke, petroleumcoke, etc.), graphite, vitreous carbon, fired organic polymers (obtainedby firing a phenolic, furane, or similar resin at an appropriatetemperature so as to carbonize it), carbon fibers, and activated carbon;lithium alloys (e.g., Li—Al alloys); and polymers such as polyacenes andpolypyrrols.

[0051] An electrolytic solution for a lithium secondary battery isusually a nonaqueous solution of a lithium salt as an electrolytedissolved in an organic solvent at a concentration of about 0.5-1.5 M.

[0052] Examples of suitable electrolytes include lithium perchlorate,lithium trifluoromethane-sulfonate, lithium tetrafluoroborate, lithiumhexafluorophosphate, lithium hexafluoroarsenate, and the like.

[0053] The organic solvent used to dissolve the electrolyte includes,for example, propylene carbonate, ethylene carbonate, butylenecarbonate, γ-butyrolactone, dimethyl carbonate, ethyl methyl carbonate,acetate compounds, propionate compounds, diacetate compounds,dimethoxyethane, diethoxyethane, dimethoxypropane, diethoxypropane,tetrahydrofuran, dioxolanes, and the like, which may be used singly oras a mixed solvent of two or more of these solvents.

[0054] In the future, it will be possible to use a polymeric electrolytewhich is now under development including a polymer complex typeelectrolyte having Li ions coordinated to oxygen atoms of a polymer toform a complex or a gel type electrolyte.

[0055] The shape and operating potential grade of the nonaqueouselectrolyte secondary battery are not critical. The battery may be ofany shape, so it may be a coin-shaped battery, cylindrical spiral-typebattery, flat rectangular battery, inside-out cylindrical battery,polymer battery, or the like. With respect to the operating potentialgrade, it is possible to constitute a battery of up to max. 5V gradedepending on the combination with the material for the negativeelectrode.

[0056] Using a positive active material according to the presentinvention, it is made possible to manufacture nonaqueous electrolytesecondary batteries having a higher capacity and a comparable or higherthermal stability with lower costs, compared to current lithium ionsecondary batteries using LiCoO₂ as a positive active material.

[0057] Furthermore, the nonaqueous electrolyte secondary batteriesaccording to the present invention can also be used suitably aslarge-sized batteries for automobiles or of the stationary type, sincethey can provide a high voltage with a high energy density and have agood cycle life at high temperatures.

EXAMPLES

[0058] The present invention will be illustrated by the followingexamples, which are presented merely for illustrative purpose and arenot intended to be restrictive in any way.

Example 1

[0059] Preparation of Compound Oxide

[0060] An aqueous solution A was prepared by dissolving nickel sulfate,manganese sulfate, and cobalt sulfate in purified water in such aproportion that the molar ratio of Ni:Mn:Co was 0.56:0.30:0.14.Separately, an aqueous solution B was prepared by adding a concentratedaqueous ammonia to an aqueous solution of ammonium hydrogen carbonate inan amount sufficient to make the solution alkaline. To an agitated tankcontaining an appropriate amount of water, the aqueous solutions A and Bwere alternatingly added in small portions at a given flow rate usingmetering pumps. After the addition was finished, the precipitates whichwere formed were collected by filtration, washed with water, and driedat 60° C. for one day to give a Ni—Mn—Co compound carbonate.

[0061] The compound carbonate was thermally decomposed by heating ataround 550° C. in air to give a Ni—Mn—Co compound oxide. The resultingcompound oxide had an average particle diameter of about 10 μm and wasdirectly used in the preparation of a positive active material withoutpulverization and classification.

[0062] Preparation of Positive Active Material

[0063] To 28.34 kg of the compound oxide, 9.16 kg of lithium hydroxide(anhydrous) and 2.5 kg of tungsten trioxide were added and uniformlymixed. The lithium hydroxide used was a fine powder obtained bypulverization in an oscillating ball mill followed by classification tocollect particles of 20 μm or smaller. The tungsten trioxide was also afine powder of 1 to 20 μm. The resulting powder mixture was placed in analuminum vessel having a purity of 99.8% and calcined for 10 hours at atemperature of 920-950° C. in a pure oxygen atmosphere to give 36.67 kgof a lithium compound oxide as a positive active material.

[0064] The composition of the resulting lithium compound oxide wasanalyzed by ICP emission spectrometry and atomic absorption spectrometryand determined to contain 7.1% Li, 30.8% Ni, 16.0% Mn, 8.3% Co, and 4.6%W in mass % and have a molar ratio of Li/(Ni+Mn+Co+W) of 1.04. Thelithium compound oxide had the composition shown in Table 1.

[0065]FIG. 1 shows an X-ray diffraction pattern of the lithium compoundoxide using Cu Kα rays. As can be confirmed from FIG. 1, the X-raydiffraction pattern includes, in addition to main diffraction peakscapable of being indexed under space group R3-m (hexagonal system)(peaks assigned to LiNiO₂ or LiCoO₂), secondary diffraction peaks at19.76°, 20.98°, and 23.54° assigned to Li₂WO₄ of space group R-3(rhombohedral system).

[0066] Testing Methods of Positive Active Material

[0067] (1) Battery Test

[0068] The lithium compound oxide (positive active material) preparedabove, acetylene black (conducting additive), and apolytetrafluoroethylene resin (binder) were placed in a mortar at a massratio of active material:conducting additive:binder of 67:22:11 andmixed for 15 minutes therein. The mixture was molded into a disc havinga thickness of 0.2 mm and a diameter of 18 mm, which was press-bonded toa stainless mesh having a diameter of 18 mm and dried at 200° C. toprovide a positive electrode to be tested.

[0069] The positive electrode prepared above, a polypropylene separator(sold under the tradename Cellguard), and a lithium metal foil measuring0.2 mm in thickness (negative electrode) were stacked in a test cell asshown in FIG. 8. As an electrolytic solution, a 1M solution of lithiumhexafluorophosphate (LiPF₆) in a mixed solvent of ethylene carbonate(EC) and dimethoxy carbonate (DMC) at a volume ratio of 1:2 was used andpoured into the test cell.

[0070] A repeated charge-discharge test was performed using acoin-shaped test battery having the above described structure. Thecharge and discharge were carried out under voltage regulations bycharging to 4.3 V and discharging to 3.0 V with a constant current of0.707 mA (0.4 mA/cm²). The discharge capacity obtained after the firstcharging under the above-described voltage regulations was recorded asan initial capacity. The charge and discharge were repeated for 50cycles under the above-described conditions, and the discharge capacityobtained in the 50th cycle was also measured. The percent retention ofthe 50th cycle discharge capacity relative to the initial capacity wascalculated to evaluate the cycle life of the test battery. The testresults are also given in Table 1.

[0071] (2) DSC Measurement (Thermal Stability in Charged State)

[0072] A coin-shaped battery was assembled in the same manner asdescribed for (1) above. After being charged to 4.3 V, the battery wasdisassembled. The disc of the positive active material in a chargedstate was recovered from the positive electrode and thoroughly washedwith dimethoxy carbonate (DMC). A 2 mg aliquot of the positive activematerial was placed into a stainless steel pressure pan for DSCmeasurement along with about 2 μL of the same electrolytic solution asabove [1M LiPF₆ in (EC+DMC, 1:2)] and DSC measurement was carried outwhile the temperature was increased from 25° C. to 500° C. at a rate of10° C./min. The results are shown in Table 1 and FIG. 7.

Example 2

[0073] To 29.04 kg of a Ni—Mn—Co compound oxide(Ni:Mn:Co=0.56:0.30:0.14) prepared in the same manner as described inExample 1, 9.39 kg of lithium hydroxide (anhydrous) and 1.57 kg ofmolybdenum trioxide were added and uniformly mixed. The lithiumhydroxide was the same fine powder of 20 μm or smaller as used inExample 1, and the molybdenum trioxide was also a fine powder of 1 to 20μm. The resulting powder mixture was placed into an aluminum vesselhaving a purity of 99.8% and calcined in the same manner as described inExample 1 to give 36.92 kg of a lithium compound oxide as a positiveactive material.

[0074] Analysis of the composition of the resulting lithium compoundoxide showed that it contained 7.2% Li, 30.9% Ni, 16.1% Mn, 8.3% Co, and2.8% Mo in mass % and had a molar ratio of Li/(Ni+Mn+Co+Mo) of 1.05. Thelithium compound oxide had the composition shown in Table 1.

[0075] An X-ray diffraction pattern of the lithium compound oxide usingCu Kα rays is shown in FIG. 2. It can be seen that the X-ray diffractionpattern includes, in addition to main diffraction peaks capable of beingindexed under space group R3-m, a secondary diffraction peak at 21.08°assigned to Li₄MoO₅ of the rhombohedral system.

[0076] The results of measurements with a test battery prepared in thesame manner as described in Example 1 and the results of DSCmeasurements are shown in Table 1 and FIG. 7.

Example 3

[0077] In the same manner as described in Example 1 except that 9.29 kgof lithium hydroxide (anhydrous) and 1.71 kg of tungsten trioxide wereadded to 29.01 kg of a Ni—Mn—Co compound oxide(Ni:Mn:Co=0.56:0.30:0.14),36.56 kg of a lithium compound oxide wereobtained.

[0078] Analysis of the composition of the resulting lithium compoundoxide showed that it contained 7.3% Li, 31.1% Ni, 16.0% Mn, 8.4% Co, and3.8% W and had a molar ratio of Li/(Ni+Mn+Co+W) of 1.07.

[0079] An X-ray diffraction pattern of the lithium compound oxide usingCu Kα rays is shown in FIG. 3. It can be seen that the X-ray diffractionpattern includes, in addition to main diffraction peaks capable of beingindexed under space group R3-m, secondary diffraction peaks at 21.04°and 23.44° assigned to Li₂WO₄ of the rhombohedral system.

[0080] The results of measurements with a test battery prepared in thesame manner as described in Example 1 and the results of DSCmeasurements are shown in Table 1 and FIG. 7.

Comparative Example 1

[0081] A mixture obtained by adding 9.56 kg of lithium hydroxide(anhydrous) to 30.44 kg of a Ni—Mn—Co compound oxide(Ni:Mn:Co=0.56:0.30:0.14) followed by mixing uniformly was calcined at atemperature of 900° C. in the same manner as described in Example 1 togive 36.92 kg of a lithium compound oxide.

[0082] Analysis of the composition of the resulting lithium compoundoxide showed that it contained 7.4% Li, 32.8% Ni, 16.2% Mn, and 8.5% Coin mass % and had a molar ratio of Li/(Ni+Mn+Co) of 1.07.

[0083] An X-ray diffraction pattern of the lithium compound oxide usingCu Kα rays is shown in FIG. 4. It can be seen that the X-ray diffractionpattern includes only main diffraction peaks capable of being indexedunder space group R3-m.

[0084] The results of measurements with a test battery prepared in thesame manner as described in Example 1 and the results of DSCmeasurements are shown in Table 1 and FIG. 7.

Comparative Example 2

[0085] A mixture obtained by uniformly mixing 32.65 kg of commerciallyavailable nickel hydroxide and 7.35 kg of lithium hydroxide (anhydrous)was calcined at a temperature of 700° C. in the same manner as describedin Example 1 to give 31.92 kg of lithium nickelate.

[0086] Analysis of the composition of the resulting lithium nickelateshowed that it contained 6.8% Li, 59.4% Ni, and 0.7% Co in mass % andhad a molar ratio of Li/(Ni+Co) of 0.96.

[0087] An X-ray diffraction pattern of the product using Cu Kα rays isshown in FIG. 5. It can be seen that the X-ray diffraction patternincludes only main diffraction peaks capable of being indexed underspace group R3-m.

[0088] The results of measurements with a test battery prepared in thesame manner as described in Example 1 and the results of DSCmeasurements are shown in Table 1 and FIG. 7.

Comparative Example 3

[0089] A mixture obtained by uniformly mixing 31.66 kg of commerciallyavailable cobalt tetraoxide and 8.34 kg of lithium hydroxide (anhydrous)was calcined at a temperature of 900° C. in the same manner as describedin Example 1 to give 36.08 kg of lithium cobaltate.

[0090] Analysis of the composition of the resulting lithium cobaltateshowed that it contained 7.0% Li and 59.7% Co in mass % and had a molarratio of Li/Co of 1.00.

[0091] An X-ray diffraction pattern of the product using Cu Kα rays isshown in FIG. 6. It can be seen that the X-ray diffraction patternincludes only main diffraction peaks capable of being indexed underspace group R3-m.

[0092] The results of measurements with a test battery prepared in thesame manner as described in Example 1 and the results of DSCmeasurements are shown in Table 1 and FIG. 7.

Comparative Example 4

[0093] In exactly the same manner as described in Example 1 except that9.56 kg of lithium hydroxide (anhydrous) and 0.88 kg of aluminumhydroxide were added to 29.57 kg of a Ni—Mn—Co compound oxide(Ni:Mn:Co=0.56:0.30:0.14), 37.12 kg of a lithium compound oxide wereobtained.

[0094] The lithium compound oxide had a compositional formula and acomposition as shown in Table 1. The results of measurements with a testbattery prepared in the same manner as described in Example 1 and theresults of DSC measurements are shown in Table 1 and FIG. 7.

Comparative Example 5

[0095] In exactly the same manner as described in Example 1 except that9.54 kg of lithium hydroxide (anhydrous) and 0.93 kg of titanium oxidewere added to 29.52 kg of a Ni—Mn—Co compound oxide(Ni:Mn:Co=0.56:0.30:0.14), 36.58 kg of a lithium compound oxide wereobtained.

[0096] The lithium compound oxide had a compositional formula and acomposition as shown in Table 1. The results of measurements with a testbattery prepared in the same manner as described in Example 1 and theresults of DSC measurements are shown in Table 1 and FIG. 7.

Comparative Example 6

[0097] To 29.84 kg of a commercially available nickel hydroxide, 8.01 kgof lithium hydroxide (anhydrous) and 2.15 kg of tungsten trioxide wereadded and uniformly mixed. The lithium hydroxide was the same finepowder of 20 μm or smaller as used in Example 1, and the tungstentrioxide was also a fine powder of 1 to 20 μm. The resulting powdermixture was placed in an aluminum vessel having a purity of 99.8% andcalcined for more than several hours at a temperature of 950-750° C. ina pure oxygen atmosphere to give 32.82 kg of a lithium compound oxide.

[0098] The lithium compound oxide had a compositional formula and acomposition as shown in Table 1. The results of measurements with a testbattery prepared in the same manner as described in Example 1 and theresults of DSC measurements are shown in Table 1 and FIG. 7. TABLE 1Metal Contents (mass %) Battery Test DSC Results Initial Cycle ExothermCalorific Other Capacity Life Peak Value No. Compositional Formula Li NiMn Co Metal (mAh/g) (%) Temp. (J/g) Example 1Li_(1.04)Ni_(0.53)Mn_(0.30)Co_(0.14)W_(0.03)O₂ 7.1 30.8 16.0 8.3 W: 4.6157 95 312° C. 295 2 Li_(1.05)Ni_(0.53)Mn_(0.30)Co_(0.14)Mo_(0.03)O₂ 7.230.9 16.1 8.3 Mo: 2.8 156 94 296° C. 366 3Li_(1.07)Ni_(0.54)Mn_(0.30)Co_(0.14)W_(0.02)O₂ 7.3 31.1 16.0 8.4 W: 3.8161 94 311° C. 363 Comparative Example 1Li_(1.07)Ni_(0.56)Mn_(0.30)Co_(0.14)O₂ 7.4 32.8 16.2 8.5 — 165 89 314°C. 619 2 Li_(0.96)Ni_(0.99)Co_(0.01)O₂ 6.8 59.4 0.0 0.7 — 180 80 220° C.1236 3 Li_(1.00)Co_(1.00)O₂ 7.0 0.0 0.0 59.7 — 150 95 250° C. 517 4Li_(1.05)Ni_(0.54)Mn_(0.30)Co_(0.14)Al_(0.03)O₂ 7.4 32.0 16.6 8.4 Al:0.8 154 89 312° C. 643 5 Li_(1.05)Ni_(0.54)Mn_(0.30)Co_(0.14)Ti_(0.03)O₂7.4 31.8 16.6 8.4 Ti: 1.4 157 92 302° C. 656 6Li_(1.08)Ni_(0.97)W_(0.03)O₂ 7.0 55.0 — — W: 5.1 137 89 216° C. 704

[0099] As can be seen from the results of the battery test shown inTable 1, the initial capacity of lithium nickelate of ComparativeExample 2 was considerably higher than that of lithium cobaltate ofComparative Example 3, which is used as a positive active material incurrent practical lithium ion secondary batteries. However, lithiumnickelate had a low DSC exotherm peak temperature of 220° C. with a verylarge calorific value of 1236 J/g in a charged state. Thus, the thermalstability of lithium nickelate in a charged state was significantlylower than that of lithium cobaltate.

[0100] In contrast, the lithium compound oxide of Comparative Example 1in which Mn and Co were added to lithium nickelate maintained an initialcapacity higher than that of lithium cobaltate and had a DSC exothermpeak temperature shifted to a higher temperature than that of lithiumnickelate with an approximately halved calorific value. Thus, thelithium compound oxide of Comparative Example 1 was improved to someextent with respect to thermal stability in a charged state. However,its calorific value was still larger than that of lithium cobaltate ofComparative Example 3, so the improvement in thermal stability wasinsufficient.

[0101] The lithium compound oxides of Examples 1 to 3, in which a verysmall amount of W or Mo at a molar ratio of 0.02 to 0.03 in theircompositional formula was added to the Mn, Cu-containing lithiumnickelate of Comparative Example 1 according to the present invention,had DSC calorific values in a charged state which were reduced toapproximately half (48-59%) the value in Comparative Example 1.Nevertheless, these oxides had initial capacities which were stillmaintained at substantially the same level as the material ofComparative Example 1, i.e., which were higher than that of lithiumcobaltate of Comparative Example 3. The DSC exotherm peak temperaturesof these oxides were nearly the same as that of Comparative Example 1.Therefore, the addition of a very small amount of W and/or Mo accordingto the present invention has the surprising thermal stabilization effectthat it is possible to approximately halve the calorific value withoutsignificantly varying the initial capacity or shifting the exotherm peaktemperature.

[0102] In accordance with the present invention, it is made possible toprovide high-performance positive active materials which are improved inboth capacity and thermal stability in a charged state with lower costsdue to a lower Co content, compared to lithium cobaltate (ComparativeExample 3) which is the current practical positive active material.Specifically, they have a higher initial capacity, a DSC exotherm peaktemperature increased by more than about 50° C., and a calorific valuesuppressed to about 57-71% compared to lithium cobaltate. Compared tolithium nickelate (Comparative Example 2), the calorific values of thepositive active materials according to the present invention in acharged state are significantly reduced to 24-30% the value of lithiumnickelate.

[0103] The exothermic heat appears to be concerned with a reactionbetween the electrolytic solution and oxygen, which is generated fromthe positive active material in a charged state by decompositionoccurring at a high temperature. With a positive active materialaccording to the present invention, it is presumed that thedecomposition temperature shifts to a higher temperature and that theamount of oxygen generated by the decomposition is significantly small.

[0104] With respect to cycle life, the positive active materials ofExamples 1 to 3 according to the present invention had an excellentcycle life comparable to that of lithium cobaltate of ComparativeExample 3 which is now in practical use. Since the cycle life of theseactive materials was improved over the material of Comparative Example 1which did not contain W and Mo, it can be concluded that the addition ofW or Mo is effective for improving not only thermal stability but alsocycle life. The positive active material of lithium nickelate ofComparative Example 2 had a poor cycle life.

[0105] As shown in FIGS. 1 to 3, the lithium compound oxides obtained inExamples 1 to 3 gave X-ray diffraction patterns including one or morepeaks assigned to the rhombohedral system of a compound oxide of Li withthe added W or Mo (Li₂WO₄, Li₄MoO₅). Thus, it can be concluded that atleast part of the added W or Mo is present as a compound oxide with Liand forms a different phase from the hexagonal phase. Although themechanism is not clear, the phase of the compound oxide of Li with W orMo seems to contribute to the improved thermal stability of a positiveactive material according to the present invention.

[0106] With the lithium compound oxides of Comparative Examples 4 and 5in which Al or Ti was added, in place of W or Mo, at the same molarratio of 0.03 as the Examples, the DSC calorific values in a chargedstate were approximately at the same level as or slightly inferior to(larger than) the value in Comparative Example 1 containing no additiveelement. Thus, the effect on improving thermal stability in a chargedstate achieved by addition of a small amount of an element to thecomposition of Comparative Example 1 in accordance with the presentinvention is inherently applicable when the added element is W or Mo andit cannot be obtained when another element is added.

[0107] The material of Comparative Example 6 in which a small amount ofW was added to lithium nickelate was not satisfactory in both batteryproperties and thermal stability. Thus, it is entirely impossible toachieve the object of the present invention by addition of W alone topure lithium nickelate which is not in a compound oxide.

[0108] Industrial Applicability

[0109] The present invention provides a high-performance positive activematerial which has a higher initial capacity and higher thermalstability in a charged state compared to LiCoO₂ (lithium cobaltate) usedas a positive active material in current practical lithium ion secondarybatteries. The positive active material according to the presentinvention can be prepared less expensively than the current positiveactive material. Therefore, the positive active material is useful invarious nonaqueous electrolyte secondary batteries including lithium ionsecondary batteries, thereby contributing to improvement in performanceand reduction in costs of nonaqueous electrolyte secondary batteries andto development of electric cars equipped with large-sized nonaqueouselectrolyte secondary batteries.

[0110] It should be understood by those skilled in the art that thepresent invention is not restricted to the specific embodimentsdescribed above and various modifications of the embodiments describedabove can be made without departing from the scope of the presentinvention.

1. A positive active material for nonaqueous electrolyte secondarybatteries, characterized in that it is comprised of a lithium compoundoxide of the formula: Li_(a)Ni_(b)Co_(c)Mn_(d)M_(e)O₂   (1) where mstands for one or two of W and Mo, 0.90≦a≦1.15, 0<b<0.99, 0<c≦0.5,0<d≦0.5, 0<c+d≦0.9, 0.01≦e≦0.1, and b+c+d+e=1, and in that the lithiumcompound oxide gives an x-ray diffraction pattern including adiffraction peak or peaks assigned to a compound oxide of Li and Wand/or a compound oxide of Li and Mo, in addition to main diffractionpeaks assigned to a hexagonal crystal structure.
 2. The positive activematerial for nonaqueous electrolyte secondary batteries according toclaim 1, wherein “a” is 0.95≦a≦1.10.
 3. The positive active material fornonaqueous electrolyte secondary batteries according to claim 1, wherein“c” is 0.05≦c≦0.4.
 4. The positive active material for nonaqueouselectrolyte secondary batteries according to claim 1, wherein “d” is0.01≦d≦0.3.
 5. The positive active material for nonaqueous electrolytesecondary batteries according to claim 1, wherein “c” and “d” are0<c+d≦0.7.
 6. The positive active material for nonaqueous electrolytesecondary batteries according to claim 1, wherein “e” is 0.01≦e<0.5. 7.The positive active material for nonaqueous electrolyte secondarybatteries according to any one of claims 1 to 6, wherein the diffractionpeak or peaks assigned to a compound oxide of Li and W and/or a compoundoxide of Li and Mo is a peak or peaks assigned to rhombohedral Li₂WO₄and/or Li₄MoO₅.
 8. A positive electrode mixture for nonaqueouselectrolyte secondary batteries predominantly comprising a positiveactive material according to claim
 1. 9. The positive electrode mixturefor nonaqueous electrolyte secondary batteries according to claim 8which further contains a binder and a conducting additive.
 10. Anonaqueous electrolyte secondary battery comprising a negative electrodecomprised of lithium metal or a substance capable of reversiblyabsorbing and desorbing Li or Li ions and a positive electrode comprisedof a mixture according to claim 8 or 9.